The present invention concerns an analytical sandwich test, in particular a test element and in particular in the form of an immunochromatographic test strip using the sandwich principle to determine N-terminal pro-brain natriuretic peptide (NT-proBNP).
NT-proBNP is a very promising marker for the diagnosis and management of heart failure. At present, the only NT-proBNP test that is available on the in-vitro diagnostic market is the fully automated Elecsys® NT-proBNP test from Roche Diagnostics which is based on a sandwich reaction with electrochemiluminescence detection. This test is designed to be used in large central laboratories and in addition to liquid reagents that have to be exactly dosed, requires a relatively complex instrument to dose the liquids and to detect the luminescence signal in order to carry out the test. A simple to use, rapid test for NT-proBNP which if needed can be evaluated visually without an evaluation instrument is presently not on the market.
In patients with acute respiratory distress it is advantageous to carry out a NT-proBNP determination as rapidly as possible in order to exclude or diagnose heart failure as a cause of the dyspnoea and to initiate appropriate treatment. Since the Elecsys® NT-proBNP test can only be carried out in a central laboratory, it is difficult to rapidly determine NT-proBNP outside the routine times. Hence, it would be particularly advantageous for the emergency ward if a rapid test were available which could be carried out directly in the emergency ward outside of routine times. This rapid test should, however, ensure the same reference ranges and cut-offs as the reference method in the central laboratory (Elecsys® NT-proBNP) in order to enable a good comparability of the results independently of the type of test that is actually carried out.
The polyclonal antibodies (PAB) used in the Elecsys® NT-proBNP test recognize a very special fraction of NT-proBNP (“native” NT-proBNP; see International Patent Application PCT/EP2004/005091 dated May 12, 2004 from Klemt et al.; according to this the test recognizes the epitopes of NT-proBNP comprising the amino acids 1-21 (AA 1-21) and 39-50 (AA 39-50)). However, it has turned out that these polyclonal antibodies are unsuitable for NT-proBNP rapid tests that use particulate labels such as colloidal gold as a label since they exhibit a high undesired variability in the signal generation due to physico-chemical interactions with the components of the rapid test (such as the support materials, matrices, etc.). This results in considerable fluctuations in the quality of the test from batch to batch.